The objective of this study is to separate the components responsible for dose-rate effects under conditions of continuous irradiation and to investigate whether the effects of continuous irradiation can be predictably reproduced by a fractionated treatment in a simple in vitro system. The dose-rate effect due primarily to the accumulation of radiation damage and repair will be studied by continuous irradiation at various dose-rates at temperatures which slow down or prevent the movement of cells through the cycle. The effects of continuous irradiation will be compared with the effects of fractionated treatments in which the same dose per cell cycle is given. Two Chinese Hamster cell lines (V79 and CHO) will be used in the proposed experiments. The results will be used to test theoretical models of continuous irradiation.